CRT displays such as video monitors and television receivers routinely include one or more components whose sole function is to protect more fragile components from arcs developed in the CRT. Typically, the solid state video amplifier which drives one or more electrodes in the CRT is the device which requires such protection.
A conventional technique for protecting a video amplifier is shown in FIG. 1. In the illustrated arrangement, a transistor 10 represents a protected video amplifier having a load resistor R.sub.L and a series impedance R.sub.s connected between the collector of the transistor 10 and the cathode of a CRT 12. This particular CRT may be of the "soft flash" type which includes an internal, aquadag coating 14 and a current limiting resistance of about 12K ohms represented by the resistors 15.
When the CRT's electron gun arcs across the gap between the cathode and the G3 electrode which is at high potential, the resistors 15 limit the typical arc to a peak current of about 1.5 amps. The arc causes the voltage at the collector of the transistor 10 to rise rapidly, whereupon a diode 16 conducts in an attempt to hold the collector voltage to a level only slightly higher than B+. If the capacitance 18 associated with the B+ supply is large enough and if the forward voltage of the diode 16 is low enough, the voltage at the collector of the video amplifier will be held to a relatively low value. A microsecond or so after the arc, the gas in a plasma tube 20 ionizes, whereupon the plasma tube acts as a short circuit which shunts to ground most of the arc-supporting energy which was stored by the capacitance associated with the aquadag 14.
For conventional CRT displays that do not require video amplifiers with wide bandwidths, the arrangement of the diode 16 and the plasma tube 20 may provide adequate arc protection so long as the series resistance R.sub.s is sufficiently high. For high frequency CRT displays, the video amplifier may need to have a bandwidth in excess of 100 MHz, and it is this need for such a wide bandwidth that gives rise to inadequacies in the conventional arc protection circuit.
When a wide bandwidth video amplifier is required, the amplifier itself must use a transistor having a fast, delicate geometry and operate with low source impedances. With reference to FIG. 1, this would mean lowering the value of the resistance R.sub.s to avoid the bandwidth rolloff caused by the combination of a relatively higher value R.sub.s in combination with the cathode input capacity of the CRT. Lowering the value of R.sub.s reduces the efficacy of the illustrated arc protection devices, particularly when more delicate, high frequency transistors are used as video amplifiers.
The problem is further aggravated by the fact that transistors with the required characteristics of high F.sub.t, low output capacity, etc. tend to have a low breakdown voltage, typically in the range of 18 to 20 volts BV.sub.ceo. To provide sufficient video drive to the CRT, typically 30 volts peak-to-peak, the transistor output devices operate near their maximum ratings and are, therefore, susceptible to avalanche breakdown and consequent failure if an arc causes them to operate beyond their maximum rating even during the one microsecond interval required to fire the plasma tube 20. For this additional reason, the arc protection circuitry of FIG. 1 is inadequate for CRT displays using wide bandwidth video amplifiers or other components which are susceptible to damage by arcs.